1. Field of the Invention
The present invention relates to a reed switch device having a pair of magnets and a reed switch.
2. Description of Related Arts
A conventional reed switch device has a magnet which may be a permanent magnet or a solenoid and a reed switch which is adapted for opening and closing a circuit in accordance with the strength of magnetic field applied thereto. The strength of the magnetic field is changed in accordance with the relative position between the magnet and the reed switch so that the reed switch is opened and closed in accordance with the distance whereby the relative position is detected.
It is often experienced that reed switches as products of the same design exhibit a fluctuation in the operation characteristics, namely, the level of the magnetic field at which the reed switch opens and closes. In addition, each reed switch has such a hysteresis that the strength of the magnetic field at which the reed switch is switched from open state to close state and the strength of the magnetic field at which the same is switched from close state to open state differ from each other.
In actually setting a reed switch device, therefore, it is necessary to adjust the strength of the magnetic field by an amount necessary to compensate for the above-mentioned fluctuation in the operation characteristics and the hysteresis. In other words, it is necessary that the relative position between the magnet and the reedswitch is adjusted to effect the above-mentioned adjustment of the strength of the magnetic field. It is also to be understood that the precision of detection of the relative position varies in accordance with the amount of variation of the relative position corresponding to the hysteresis. The amount of variation in the relative position necessary for the compensation for the fluctuation and hysteresis depends on the rate of change in the magnetic field strength in the direction of variation in the relative position. More specifically, the greater the rate of change in the magnetic field, the smaller the variation in the relative position.
Unfortunately, in known reed switch devices, the rate of change in the strength of the magnetic field in the direction of variation in the relative position is rather small, so that the relative position has to be varied largely in order to compensate for the fluctuation in the operation characteristics and for the hysteresis of operation. This makes it difficult to reduce the size of the reed switch device to a satisfactorily level and to impede improvement in the precision of detection.